AC/DC Power Factor Corrected (PFC) converters are usually realized by using a full bridge rectifier followed by a boost converter to control the input current waveform to be sinusoidal and in phase with the input voltage waveform and then by an isolated DC/DC converter to provide galvanic isolation and output voltage regulation. This approach has been shown to be effective and efficient in both low power and higher power applications but involves several conversion stages which increases the cost. Also the boost converter stage does not have output current limiting necessitating an additional circuit to limit the inrush current.
Many authors have proposed single stage PFC isolated converters using Flyback or SEPIC topologies such as Q. Zhang in “A New Digital Controller for a Single Stage Bi-flyback PFC Converter”, INTELEC 2010, Orlando Fla., June 2010 and H. Choi in “Two Switch BCM Single-Stage PFC for HB LED Lighting Applications”, APEC 2013, Long Beach, Calif., March 2013 and K. Reanzi et al. in “A New Control Scheme for an AC/DC Single-Stage Buck-Boost Converter with Improved Output Ripple Reduction and Transient Response” APEC 2014, Fort Worth Tex., March 2014 and D. S. L. Simonetti et al. in “Design Criteria for SEPIC and Cuk converters as Power Factor Pre-regulators in Discontinuous Conduction Mode”. These single stage PFC isolated converters are suitable only for lower output powers and need additional circuit modifications to achieve reasonably low input current Total Harmonic Distortion (THD). To allow efficient operation at higher output power and to realize low input current THD it would be best if a boost derived topology was used due to lower component load factor as suggested by Bruce Carsten in “Converter Component Load Factors: A Performance Limitation of Various Topologies”, PCI '88, Munich W. Germany, December 88. Also it would be best if the topology used features Zero Voltage Switching (ZVS) to allow high frequency operation, to maximize conversion efficiency and to minimize Electromagnetic Interference (EMI) generation.
One known isolated boost topology that could be used to realize single stage isolated PFC is the Clarke Converter as disclosed by Patrick William Clarke in U.S. Pat. No. 3,938,024, Feb. 10 1976. This topology is the integration of the boost converter with a 50% push pull converter and operates by having the conduction time of the two primary switches overlapping. However overload and short circuit current limiting is difficult to achieve and a second winding in the inductor is required to absorb the input inductor energy should both switching devices be turned off at the same time, which impresses extra voltage on the switching devices.
Another isolated boost converter topology, previously invented by the author and known as the Davidson Converter, is described in U.S. Pat. No. 4,559,590. This converter is realized by adding a simple 2 winding transformer T with primary and secondary DC blocking capacitors Cp and Cs, a small valued resonant inductor Lr and a diode D2 to the basic non-isolated boost regulator circuit as shown in FIG. 3.
The transformer T provides galvanic isolation and voltage conversion depending upon its turns' ratio Ns/Np and the diode D2 provides a path to recharge the DC blocking capacitors when the primary switch S1 is conducting. A resonant inductance Lr, which can be located in series with the primary or secondary or be the transformer leakage inductance, conducts and limits the discharge current through S1 and D2 when S1 is on. The resonant frequency of Lr and the series combination of Cp and Cs can be selected to be higher or lower than the switching frequency of the converter so that the resonant half cycle of current is discontinuous during the time that S1 is on or is continuous.
An aspect of the presence of the resonant inductor Lr is that it limits the energy that can be transferred from the primary to the secondary when the duty cycle of switch S1 is low so that current limiting and short circuit protection can be provided by this converter.
One problem introduced by the resonant inductor Lr when it is connected effectively in series with the current path when S1 is off (and by the transformer leakage inductance) is that the voltage across switch S1 will have a large overvoltage due to the inductor L driving current into these smaller inductances. Accordingly an active clamp, as invented by Bruce Carsten for the forward converter is described in “High Power SMPS Require Intrinsic Reliability”, PCI'81 proceedings, September 1981, Munich, Germany, consisting of auxiliary switch S2 and clamp capacitor Cclamp was included to clamp the turn off voltage of the main switch S1 as shown in FIG. 4 of this application. The clamp circuit also introduces an additional benefit in that with suitable values of Lr and L, ZVS of both the main switch S1 and the clamp switch S2 can be achieved for all loads and controlled dI/dT turn off of the output rectifying diodes is realized. A clamp circuit really is an essential part of the topology as the energy lost without it is considerable and the active clamp which returns stored energy to converter directly is the best type of clamp to use.
An alternate version of the clamp circuit, the half bridge clamp, can be derived by reversing the positions of S2 and Cclamp and then sliding blocking capacitor Cp through to the other side of the winding then connecting Cclamp to the other side of the blocking capacitor in a similar fashion to that described elsewhere in the patent. Alternate versions of the secondary rectification circuit, such as that shown in FIG. 5 which uses a ½ bridge of capacitors Cs1 and Cs2 rather than the single capacitor Cs can also be used. This version works in the same fashion as the previous rectification circuit in that its output voltage is equal to the peak to peak voltage of the input waveform from the transformer secondary.
Another version of the output circuit that was not described in the original patent is the full wave bridge version. A blocking capacitor is still needed to allow operation over varying duty cycles. In fact any output rectification configuration can be used, as long as the output voltage generated is equal to or a multiple of the peak to peak voltage of the secondary winding, without changing the fundamental nature of the converter topology.
One unique and very interesting property of this topology is that it can be used to create outputs of either polarity without changing the phasing of the secondary winding(s) or create two outputs of either polarity with one secondary winding. The polarity of the output diodes is simply reversed. Similarly the converter can accept either polarity of input voltage and create one polarity of output voltage if the polarity of the primary side switches is reversed. These unique properties hold only if the resonant inductance is in series with the primary and/or secondary windings (or is the transformer leakage inductance) and are not in series with one of the output diodes. These properties allow the converter topology to potentially accept both polarities of input voltage.
Other authors have explored using the Davidson Converter for Isolated PFC Converters although they did not properly refer to it as such. Slobodan Cuk in “Single-Stage Bridgeless Isolated PFC Converter Achieves 98% Efficiency”, Power Electronics Technology, October 2010 proposed using a version of the Davidson Converter without an active clamp circuit. Accordingly this converter suffers from high losses as the energy stored in the resonant inductance is not recovered. Muntasir Alam et al. in “A Single-Stage Bridgeless High Efficiency ZVS Hybrid-Resonant Off-Road and Neighborhood EV Battery Charger” APEC 2014 proposes using a totem pole version of the Davidson Converter with an active clamp. This version is not a true bridgeless PFC converter. S. Nigsch et al. in “Analysis, Modelling and Design of a True Bridgeless Single Stage PFC with Galvanic Isolation”, APEC2015, Charlotte N.C., March 2015 proposes using an inferior and complicated clamp circuit with two MOSFETs with series connected diodes which is difficult to control and does not allow Zero Voltage Switching.
It is accordingly an object of the invention is to provide an improved single stage isolated power factor converter which is suitable for high power operation, features Zero Voltage Switching to maximize conversion efficiency and to minimize electromagnetic interference (EMI) generation, does not need an additional circuit to limit the inrush current, achieves reasonably low input current THD, and is easy to control. It is a further object of the invention to provide a true bridgeless single stage isolated power factor converter with even higher efficiency and lower input current THD.
These and other objects of the invention will be better understood by reference to the detailed description of the preferred embodiment which follows. Note that the objects referred to above are statements of what motivated the invention rather than promises. Not all of the objects are necessarily met by all embodiments of the invention described below or by the invention defined by each of the claims.